The present invention relates, in general, to deflecting radiated energy, and more particularly, to supressing a portion of deflected radiated energy.
In many applications of image production, light from an image source is diffused and diffracted off line to a viewing area. This image-light directional change is often effected using a diffraction optics directional diffusing screen such as disclosed in U.S. Pat. No. 4,372,639 which diffuses and directs most of the light impinging thereon into a well-defined exit pupil. However, some of the light incident on the screen passes through that screen in an undiffracted manner. This undiffracted light is known as zero-order light and presents many problems.
For example, any zero-order light in the direction of the viewer's exit pupil causes an intense "hot spot" to be observed on the screen, which is visually blinding and reduces viewability of the remaining screen, and is therefore totally unacceptable. Further, to prevent zero-order light from entering the exit pupil, the position of the exit pupil is restricted to areas outside the volume bathed in zero-order light from the image source. The result is either an off-axis exit pupil, which may be undesirable to the viewer, or an off-axis projection onto the screen, which causes problems with image distortion and uneven illumination of the screen. Zero-order light outside of the exit pupil is undesirable because it contributes to the ambient lighting in a closed environment, such as a cockpit, and thus may indirectly degrade the viewability of the screen, and zero-order light from an intense light source, such as a mercury arc lamp, may cause temporary flashblindness or even permanent eye damage to the viewer inadvertently looking into it.
Therefore, while directional diffusing screens have many advantageous properties, the zero-order light associated with such screens produces many problems. It would therefore be very desirable to be able to remove zero-order light from any viewing area associated with such screens.
It is known to separate zero-order light from incident light, see, for example, U.S. Pat. No. 3,544,711, and devices such as disclosed in U.S. Pat. No. 3,980,389 use diffraction gratings to deflect a beam of incident light. However, these devices are not disclosed in a manner suggesting any use with directional diffusing screens.
Mechanical devices have been used to perform an operation on a zero-order beam. Thus, a zero-order beam is focused through an aperture in the device disclosed in U.S. Pat. No. 4,104,489.
However, the zero-order beam in U.S. Pat. No. 4,104,489 is not removed, but is focused onto a recording disc to be used in forming an interference pattern and thus this device is not suggestive of the above-discussed problems incident directional diffusing screen. Mechanical means which include grids and vanes are disclosed in U.S. Pat. No. 3,580,655 for the purpose of blocking off rays from a zero-order image.
However, mechanical means, such as those disclosed in U.S. Pat. No. 3,580,655 have low transmission capabilities whereby part of the light will be blocked thereby reducing display image intensity. Furthermore, such mechanical means have small viewing angles and thus tend to block out divergent rays, or rays at angles beyond the designed viewing angle. In fact, such mechanical means may operate on the light rays differently depending on the angle of incidence between those rays and the means. Thus, a viewed image may have different perceived brightness and resolution for different portions of the viewing area.
These mechanical means have a significant effect on any light passing through them. Therefore, if the light which is desirably focused on a viewing area passes through such mechanical means, such desirable light will also be affected by the means meant to operate only on the zero-order light. The directional nature of such mechanical means therefore may make perceived brightness and resolution dependent on the viewer's position with respect to the viewing area. This may make it difficult for a viewer to observe all parts of the viewing area and reliably extract information regardless of his position or orientation with respect to that viewing area.
The directional nature of the mechanical means, such as disclosed in the aforementioned U.S. Pat. No. 4,104,489, may make such devices fully effective at only a single angle, or within only a very limited range of angles with light at angles different from this effective range passing through unaffected by the device. Therefore, multiple angle operation may be impossible, and a transition from design effectiveness to total uneffectiveness being gradual. Such gradual transition may degrade the reliability of a device using such mechanical means.
Therefore, the prior art does not provide any device which is capable of providing advantages of a directional diffusing screen but which does not have the disadvantages inherent in zero-order light.